Thermal characterization of porous silicon via thermal wave interferometry

The photoacoustic technique is applied to study the thermal properties of n-type porous silicon samples having different porosity. The photoacoustic response of the investigated samples is referenced to samples having the same porosity but larger thickness and analysed as a function of the modulation frequency of the incident absorbed light. The response points out interference effects due to multiple reflections of the excited thermal wave inside the thin layer of the porous material. Taking into account also the contribution to the signal of the internal gas expansion, an estimation of the thermal conductivity of the porous samples is derived. The measured thermal conductivity is found to be lower than that reported for crystalline silicon by two orders of magnitude. (C) 1999 Elsevier Science B.V. All rights reserved.